1 .. SPDX-License-Identifier: GPL-2.0
3 ============================================================
4 Linux kernel driver for Elastic Network Adapter (ENA) family
5 ============================================================
10 ENA is a networking interface designed to make good use of modern CPU
11 features and system architectures.
13 The ENA device exposes a lightweight management interface with a
14 minimal set of memory mapped registers and extendible command set
15 through an Admin Queue.
17 The driver supports a range of ENA devices, is link-speed independent
18 (i.e., the same driver is used for 10GbE, 25GbE, 40GbE, etc), and has
19 a negotiated and extendible feature set.
21 Some ENA devices support SR-IOV. This driver is used for both the
22 SR-IOV Physical Function (PF) and Virtual Function (VF) devices.
24 ENA devices enable high speed and low overhead network traffic
25 processing by providing multiple Tx/Rx queue pairs (the maximum number
26 is advertised by the device via the Admin Queue), a dedicated MSI-X
27 interrupt vector per Tx/Rx queue pair, adaptive interrupt moderation,
28 and CPU cacheline optimized data placement.
30 The ENA driver supports industry standard TCP/IP offload features such as
31 checksum offload. Receive-side scaling (RSS) is supported for multi-core
34 The ENA driver and its corresponding devices implement health
35 monitoring mechanisms such as watchdog, enabling the device and driver
36 to recover in a manner transparent to the application, as well as
39 Some of the ENA devices support a working mode called Low-latency
40 Queue (LLQ), which saves several more microseconds.
41 ENA Source Code Directory Structure
42 ===================================
44 ================= ======================================================
45 ena_com.[ch] Management communication layer. This layer is
46 responsible for the handling all the management
47 (admin) communication between the device and the
49 ena_eth_com.[ch] Tx/Rx data path.
50 ena_admin_defs.h Definition of ENA management interface.
51 ena_eth_io_defs.h Definition of ENA data path interface.
52 ena_common_defs.h Common definitions for ena_com layer.
53 ena_regs_defs.h Definition of ENA PCI memory-mapped (MMIO) registers.
54 ena_netdev.[ch] Main Linux kernel driver.
55 ena_ethtool.c ethtool callbacks.
56 ena_pci_id_tbl.h Supported device IDs.
57 ================= ======================================================
62 ENA management interface is exposed by means of:
64 - PCIe Configuration Space
66 - Admin Queue (AQ) and Admin Completion Queue (ACQ)
67 - Asynchronous Event Notification Queue (AENQ)
69 ENA device MMIO Registers are accessed only during driver
70 initialization and are not used during further normal device
73 AQ is used for submitting management commands, and the
74 results/responses are reported asynchronously through ACQ.
76 ENA introduces a small set of management commands with room for
77 vendor-specific extensions. Most of the management operations are
78 framed in a generic Get/Set feature command.
80 The following admin queue commands are supported:
82 - Create I/O submission queue
83 - Create I/O completion queue
84 - Destroy I/O submission queue
85 - Destroy I/O completion queue
91 Refer to ena_admin_defs.h for the list of supported Get/Set Feature
94 The Asynchronous Event Notification Queue (AENQ) is a uni-directional
95 queue used by the ENA device to send to the driver events that cannot
96 be reported using ACQ. AENQ events are subdivided into groups. Each
97 group may have multiple syndromes, as shown below
101 ==================== ===============
103 ==================== ===============
104 Link state change **X**
106 Notification Suspend traffic
107 Notification Resume traffic
109 ==================== ===============
111 ACQ and AENQ share the same MSI-X vector.
113 Keep-Alive is a special mechanism that allows monitoring the device's health.
114 A Keep-Alive event is delivered by the device every second.
115 The driver maintains a watchdog (WD) handler which logs the current state and
116 statistics. If the keep-alive events aren't delivered as expected the WD resets
117 the device and the driver.
122 I/O operations are based on Tx and Rx Submission Queues (Tx SQ and Rx
123 SQ correspondingly). Each SQ has a completion queue (CQ) associated
126 The SQs and CQs are implemented as descriptor rings in contiguous
129 The ENA driver supports two Queue Operation modes for Tx SQs:
132 In this mode the Tx SQs reside in the host's memory. The ENA
133 device fetches the ENA Tx descriptors and packet data from host
136 - **Low Latency Queue (LLQ) mode or "push-mode":**
137 In this mode the driver pushes the transmit descriptors and the
138 first 128 bytes of the packet directly to the ENA device memory
139 space. The rest of the packet payload is fetched by the
140 device. For this operation mode, the driver uses a dedicated PCI
141 device memory BAR, which is mapped with write-combine capability.
143 **Note that** not all ENA devices support LLQ, and this feature is negotiated
144 with the device upon initialization. If the ENA device does not
145 support LLQ mode, the driver falls back to the regular mode.
147 The Rx SQs support only the regular mode.
149 The driver supports multi-queue for both Tx and Rx. This has various
152 - Reduced CPU/thread/process contention on a given Ethernet interface.
153 - Cache miss rate on completion is reduced, particularly for data
154 cache lines that hold the sk_buff structures.
155 - Increased process-level parallelism when handling received packets.
156 - Increased data cache hit rate, by steering kernel processing of
157 packets to the CPU, where the application thread consuming the
159 - In hardware interrupt re-direction.
164 The driver assigns a single MSI-X vector per queue pair (for both Tx
165 and Rx directions). The driver assigns an additional dedicated MSI-X vector
166 for management (for ACQ and AENQ).
168 Management interrupt registration is performed when the Linux kernel
169 probes the adapter, and it is de-registered when the adapter is
170 removed. I/O queue interrupt registration is performed when the Linux
171 interface of the adapter is opened, and it is de-registered when the
174 The management interrupt is named::
176 ena-mgmnt@pci:<PCI domain:bus:slot.function>
178 and for each queue pair, an interrupt is named::
180 <interface name>-Tx-Rx-<queue index>
182 The ENA device operates in auto-mask and auto-clear interrupt
183 modes. That is, once MSI-X is delivered to the host, its Cause bit is
184 automatically cleared and the interrupt is masked. The interrupt is
185 unmasked by the driver after NAPI processing is complete.
190 ENA driver and device can operate in conventional or adaptive interrupt
193 **In conventional mode** the driver instructs device to postpone interrupt
194 posting according to static interrupt delay value. The interrupt delay
195 value can be configured through `ethtool(8)`. The following `ethtool`
196 parameters are supported by the driver: ``tx-usecs``, ``rx-usecs``
198 **In adaptive interrupt** moderation mode the interrupt delay value is
199 updated by the driver dynamically and adjusted every NAPI cycle
200 according to the traffic nature.
202 Adaptive coalescing can be switched on/off through `ethtool(8)`'s
203 :code:`adaptive_rx on|off` parameter.
205 More information about Adaptive Interrupt Moderation (DIM) can be found in
206 Documentation/networking/net_dim.rst
210 The rx_copybreak is initialized by default to ENA_DEFAULT_RX_COPYBREAK
211 and can be configured by the ETHTOOL_STUNABLE command of the
217 The user can obtain ENA device and driver statistics using `ethtool`.
218 The driver can collect regular or extended statistics (including
219 per-queue stats) from the device.
221 In addition the driver logs the stats to syslog upon device reset.
226 The driver supports an arbitrarily large MTU with a maximum that is
227 negotiated with the device. The driver configures MTU using the
228 SetFeature command (ENA_ADMIN_MTU property). The user can change MTU
229 via `ip(8)` and similar legacy tools.
234 The ENA driver supports:
236 - IPv4 header checksum offload
237 - TCP/UDP over IPv4/IPv6 checksum offloads
242 - The ENA device supports RSS that allows flexible Rx traffic
244 - Toeplitz and CRC32 hash functions are supported.
245 - Different combinations of L2/L3/L4 fields can be configured as
246 inputs for hash functions.
247 - The driver configures RSS settings using the AQ SetFeature command
248 (ENA_ADMIN_RSS_HASH_FUNCTION, ENA_ADMIN_RSS_HASH_INPUT and
249 ENA_ADMIN_RSS_INDIRECTION_TABLE_CONFIG properties).
250 - If the NETIF_F_RXHASH flag is set, the 32-bit result of the hash
251 function delivered in the Rx CQ descriptor is set in the received
253 - The user can provide a hash key, hash function, and configure the
254 indirection table through `ethtool(8)`.
262 :code:`ena_start_xmit()` is called by the stack. This function does the following:
264 - Maps data buffers (``skb->data`` and frags).
265 - Populates ``ena_buf`` for the push buffer (if the driver and device are
267 - Prepares ENA bufs for the remaining frags.
268 - Allocates a new request ID from the empty ``req_id`` ring. The request
269 ID is the index of the packet in the Tx info. This is used for
270 out-of-order Tx completions.
271 - Adds the packet to the proper place in the Tx ring.
272 - Calls :code:`ena_com_prepare_tx()`, an ENA communication layer that converts
273 the ``ena_bufs`` to ENA descriptors (and adds meta ENA descriptors as
276 * This function also copies the ENA descriptors and the push buffer
277 to the Device memory space (if in push mode).
279 - Writes a doorbell to the ENA device.
280 - When the ENA device finishes sending the packet, a completion
282 - The interrupt handler schedules NAPI.
283 - The :code:`ena_clean_tx_irq()` function is called. This function handles the
284 completion descriptors generated by the ENA, with a single
285 completion descriptor per completed packet.
287 * ``req_id`` is retrieved from the completion descriptor. The ``tx_info`` of
288 the packet is retrieved via the ``req_id``. The data buffers are
289 unmapped and ``req_id`` is returned to the empty ``req_id`` ring.
290 * The function stops when the completion descriptors are completed or
291 the budget is reached.
296 - When a packet is received from the ENA device.
297 - The interrupt handler schedules NAPI.
298 - The :code:`ena_clean_rx_irq()` function is called. This function calls
299 :code:`ena_com_rx_pkt()`, an ENA communication layer function, which returns the
300 number of descriptors used for a new packet, and zero if
301 no new packet is found.
302 - :code:`ena_rx_skb()` checks packet length:
304 * If the packet is small (len < rx_copybreak), the driver allocates
305 a SKB for the new packet, and copies the packet payload into the
308 - In this way the original data buffer is not passed to the stack
309 and is reused for future Rx packets.
311 * Otherwise the function unmaps the Rx buffer, sets the first
312 descriptor as `skb`'s linear part and the other descriptors as the
315 - The new SKB is updated with the necessary information (protocol,
316 checksum hw verify result, etc), and then passed to the network
317 stack, using the NAPI interface function :code:`napi_gro_receive()`.